Still cameras, video cameras and cameras for mobile phones, for example, are designed to pass an image of an object to be photographed through a lens, to project it onto imaging elements such as solid-state imaging elements or charged coupled devices (CCD), or complementary metal-oxide semiconductor (CMOS) integrated circuits. In order to project a focused image onto the imaging elements, it is necessary to adjust the distance between the lens and the imaging element, in accordance with the distance between the lens and the object to be photographed.
In still cameras and video cameras, in order to focus, the distance between the lens and the imaging element is adjusted by mechanically moving the lens using a small motor, for example. By comparison, it is difficult to provide micro cameras mounted in mobile phones with a mechanical mechanism so as to move the lens, due to space restraints. Therefore, wide angle lenses that have a large focal depth are used to eliminate the need for focusing. However, even on cameras for mobile phones, there is a demand for a function for photographing objects not only at a large distance, but also at a distance that is on the order of a few centimeters. Such a function cannot be satisfied by current wide angle lenses, and thus there is a need for a focusing mechanism for small cameras.
There are many methods for moving a lens without using a motor, and of these, a method that utilizes piezo-electric elements is both simple and capable of miniaturization. When an electric field is applied to a piezo-electric member in a predetermined direction, the electric dipole moment of the piezo-electric member changes, and the piezo-electric member expands and contracts accordingly. This expansion and contraction can be utilized to move a lens.
In addition to this, a focusing method in which the lens shape is deformed to change the focal length, has been proposed.
In JP H8-114703A, a lens is described in which at least one side of the lens is provided with a pressure chamber, constituted by a transparent elastic film, into which operating oil is sealed, and in which the focal length can be controlled by deforming the transparent elastic film through pressure generated by the operating fluid acting on the transparent elastic film. In this lens, while the deformed shape of the transparent elastic film is optimized so as to minimize generation of lens aberration, the pressure of the operating oil within the pressure chamber is measured with a pressure sensor that is formed from the transparent elastic film, and thus by controlling the pressure of the operating oil according to the measured value, it is also possible to suppress changes in the focal length due to expansion and contraction of the operating oil.
In JP H11-133210A, a variable focus lens is described wherein an electric potential applied between a first electrode and a conductive elastic plate generates suction force due to coulomb's force, narrowing the interval between the two bodies, and as a result, the volume of transparent liquid expelled from the interval between the two is used to protrude convexly and deform a central portion of the transparent elastic plate with the transparent liquid at its back.
In Tokuhyo 2001-519539, a variable focus lens that uses the electro-wetting effect has been proposed. A water-repellant transparent insulating film is formed on a transparent electrode, and by arranging a liquid droplet on that, the liquid droplet is used as the lens. When a voltage is applied between a lower electrode and the liquid droplet, an electric charge is generated at the liquid/insulating film interface, and the shape of the liquid droplet is changed due to a reduction in the interfacial tension between the liquid/insulating film. A lens whose focal length is variable is realized by this change.
For a method for moving a lens using a piezo-electric member, the structure is simple so the size can be reduced. For example, if an electric field is applied between two faces of a ferro-electric member that has been processed into a sheet, if the electric dipole moment of the piezo-electric member is perpendicular to this surface, then the piezo-electric member will expand and contract in the direction of this surface. For example, using LiNbO3 as a ferro-electric member, if the sheet thickness is 0.1 mm, and the sheet has an area that is 10 mm wide by 10 mm long, then in order to contract the sheet 2 μm in the direction of the surface, it is necessary to apply a voltage of close to 1000 V. By properly designing the ferro-electric member whose piezo-electric constant is high, and the shape thereof, it is possible to lower the applied voltage to a certain degree, but it is generally necessary to apply a voltage of several hundred volts to the piezo-electric member. It is very difficult to apply this kind of voltage in a device such as a mobile phone.
In the methods of JP H8-114703A, JP H11-133210A and Tokuhyo 2001-519539, the voltage that is applied can be smaller than when using a piezo-electric body. However, in such methods it is difficult to control the shape of the lens accurately, and it is difficult to form clean images that have no distortion. Furthermore, in Tokuhyo 2001-519539, the shape of the lens is spherical due to the surface tension of the liquid. In order to take sharper images, it is necessary that the lens shape is a non-spherical hyperbolic curve-shape, but this is problematic in the method of Tokuhyo 2001-519539.